I’d say that warning them that they are singing the devils tune may help. Then you might warn them that the devil is a sadist who likes to torture sadists himself.
At some point he or she will need God’s help and salvation, better for that person to start on that road sooner rather than later. God sees all and hears all, no sin goes without some degree of punishment, either here or later. The fear of God is a healthy and efficacious thing to have, but he has mercy on a contrite heart.
If they can understand what unconditional love is then you might also explain how showing respect and love for our fellow human beings is the right way to go, treat them as he or she would wish to be treated.